GIMP – GIMP is a well known free alternative to Photoshop with similar features

Mischief Free Version – With six essential brushes, a basic palette of colors and an infinite vector canvas, Mischief-Free is perfect for hashing out ideas, life drawing, and endless doodles

MyPaint – MyPaint is a fast and easy open-source graphics application for digital painters, and comes with 39 different brushes

Verve Painter– Verve is a small painting application that uses fluid dynamics and brushes to push the paint around, though it is still in development.

Inkscape– Inkscape is an open-source professional quality vector graphics software with sophisticated drawing tools with capabilities comparable to Adobe Illustrator.

FireAlpaca – A similar program to Paint Tool Sai that offers features such as a snap feature which creates a variety of perspective overlays.

MediBang Paint – A free lightweight painting program for Windows, Mac Os and Android with cloud saving to allow users to easily switch between platforms.

SpeedyPainter – Basic painting program with a simple layout and features such as perspective grids and reference view mode

3D Modelling

Google Sketchup – Drawing-based tool for architects, designers, builders, makers and engineers who design for the physical world. SketchUp Make is a free version and SketchUp Pro is a paid version with additional functionality.

Sculptris– A free, introductory digital sculpting tool, a great stepping stone for digital sculptors, created by the makers of Zbrush.

Blender – A powerful application with full-fledged professional tools, Blender has a wide community and resources to help you learn.

SculptGL– This handy modelling program works online in your browser and is compatible with wacom tablets

Animation

Synfig – Synfig Studio is a free and open-source 2D animation software, designed as powerful industrial-strength solution for creating film-quality animation using a vector and bitmap artwork.

EmoFuri – EmoFuri is a new animation software that helps artists easily animate photoshop illustrations in a 2D-3D style.It uses PSD files of character illustrations to animate them.

Pencil2D– Pencil2D is an open-source animation/drawing software that lets you create traditional hand-drawn animation using both bitmap and vector graphics.

Live2D– This increasingly popular 2D-3D animation software is similar to Emofuri with sample models available to experiment with, and is also available in english.

Pixel Art

Piskel– An online pixel art app that lets you animate pixel art with a live preview of your work.

Piq – While it doesn’t have the best looking layout, this online tool has lots of interesting features including a colour-drift tool that’s great for making colour palettes.

Etc

Thisissand – An online fun and unique playground for creating colourful sandscapes, also comes in an app version

Flame Painter – A free demo of the full program, it allows you to try out the flame generator and change different brush settings and paint your own flame paintings